Allied Wire and Cable Shielding Overview

Shielding refers to the metallic layer surrounding a cable’s conductor, created to limit signal interference between the wire and external fields.
Shielding plays an essential role in maximizing the effectiveness of cable systems.

Types of Signal Interference

Shielding is designed to minimize signal leakage and the reception of signals produced by external sources. The electrostatic interference prevented
by shielding is also known as noise. An appropriate shield must be chosen in response to which kind of noise a cable will be exposed.

Static noise: This is distortion caused by an electrical field that has been created by another voltage source and that has
coupled into the signal-bearing circuit. Static Noise can be prevented by shielding the full circuit. Foil shields are the most effective in protecting
against this type of noise, but must cover both the transmitting and receiving ends of the circuit in order to reduce high levels of interference. The
shield must also be grounded to deliver the best results.

Magnetic noise: Magnetic fields radiated by power wiring used in different mechanical applications cause this type of interference.
The magnetic fields can create opposing current flows, disrupting the regular current flow of a wire. Twisting the cable elements is the simplest and most
effective way to combat magnetic noise.

Common mode noise: Currents flowing from different potential grounds at different points within a system cause this type of
interference. Receivers with high common mode rejection ratios can reduce the effects of common mode noise.

Crosstalk: When signals (pulsed DC or standard AC) from one wire are superimposed onto another wire nearby, it is called
crosstalk. Individual cable pair shielding coupled to pair twist is the best way to prevent crosstalk, though pair twists alone also reduce
this type of interference.

Once the type of noise is identified, appropriate materials and styles of shield can be chosen.

Common Shield Materials

Shields are available in a variety of conductive materials:

Bare copper

Tinned copper

Galvanized steel

Stainless steel

Aluminum foil

Fiberglass

Kevlar

Of these materials, aluminum and copper are the most common. Conductive nylon tapes, plastics, and textiles can also be used to minimize
signal interference, but are less effective and less common than most other shielding materials.

Shields are also available in a number of styles which can be chosen to accommodate specific environments, desired wiring characteristics and other shielding needs.

Shield Types

Foil: This shield consists of aluminum foil laminated to either a polyester or polypropylene film, giving it mechanical
strength and extra insulation. Foil provides 100% cable coverage, making it an ideal electrostatic shield. Foil shields are also lightweight,
less expensive, and more flexible than braid or spiral shields. However, they have a shorter flex life and lower mechanical strength than either
braid or spiral shields. Many times, foil is used to shield pairs of multi-pair data cables to limit crosstalk.

Braid: Woven copper or aluminum strands form braid shields. The braided structure keeps this shield structurally
sound and flexible while maintaining a long flex life. Usually braid shields cover 80% to 95% of a wire. Braids can not provide 100% coverage.
Other disadvantages of braids include bulkiness and weight. They are also more difficult to terminate than other shields because they must be
combed out. Braid shields are most commonly used to minimize low frequency noise.

Spiral: Conductive wire wraps around and up the central cable to form this shield. Benefits of spirals include flexibility,
long flex life, and up to 97% coverage. Spirals are most commonly used in audio applications. Unfortunately, they are generally ineffective above
the audio frequency range.

Combination: Some cables have both foil and braid shielding, referred to as combination shielding. Combination shielding is used to
provide the benefits of both types of shield, mainly the 100% coverage of the foil shield and the physical strength and low frequency resistance of the
braid.

Here are some other terms used in the discussion of shielding:

Shield Coverage: The percentage of wire that is physically covered by metallic shielding

Shield Effectiveness: The ability of a shield to prevent signal interference

Choosing the correct shield type, material, and amount of coverage is important to maximize the productivity of cable systems. The
environment in which the cable will be used, the potential sources of interference around the cable and the mechanical characteristics
that the cable or wire must maintain are all important elements to consider when designing a shield. The appropriate shield will minimize
most interference and ensure productive signal communication within your cable systems.